The most important think you need to know about Sudrai (called Sudraal in some records, and referred to by local farmers as Sudderly) is that while this sprawling city is fixed in time and space like any normal place, its harbor is not. On any given day most of the ships along the docks of Sudrai are perfectly ordinary medieval merchant cogs and fishing boats, but scattered among them will be a handful of vessels from other times and places. For example, some of the most exalted families of the city trace their lineages to refugees from the destruction of Atlantis and the sinking of Lyonesse, but ships from those lost civilizations make regular visits to Sudrai. In the spring the swan ships of the Sidhe will often glide into port, the eldritch barges of Melniboné have been known to trade at Sudrai (often en route to even more enigmatic destinations), and, on certain nights when the moon is full, silent galleys manned by turbanned satyrs will drift down out of the sky.
How this happens is a great mystery, but the why of it is well-established: mithril. All other known mithril mines are controlled by tight-fisted dwarves who keep all of it locked away in underground treasuries or deep-time elves who think nothing of spending a millennia or two properly smelting it to absolute purity. The tiny trickle of mithril coming out of Sudrai's mines represents the only known source of the stuff available to the highest bidder. Thus goods and gold from all over the multiverse pour into Sudrai, creating a glittering metroplis in the middle of what is otherwise an unremarkable island in a crapsack corner of a dung-ridden medieval world. Peasant farmers living within spitting distance of the walls of Sudrai suffer the same miserable existence as they do everywhere else. The citizens of Sudrai explain this is because the magic of the harbor extends only so far, but probably they're just selfish bastards who keep all the loot for themselves.